Have a Keithley 2010 on my bench that is somewhat noisy. When set to slow update rate, voltage readings seem to fluctuate up and down then back up again over a period of about 10 seconds or so. Happens whether warm or cold.
When reading the following voltages from my EDC 521 DC voltage calibrator:
Set to 100VDC, about 7mV of noise.
Set to 10VDC, about 700uV noise.
Set to 1VDC, about 70uV noise.
So noise scales with range.
It's not the calibrator, My Keithley 2000 reads fine.
The LM199/399 voltage reference is rock steady.
Worst case AC ripple across the bulk capacitors is about 200mV. Don't think that's it...
And of course no schematics.
I think this would be a fairly easy repair with them.